The invention relates generally to antireflection articles of manufacture. More particularly, the invention concerns polymeric materials modified with nanoparticulate fillers resulting in an improved optical performance.
The use of coatings to reduce the reflection from optical surfaces, such as ophthalmic lenses, is well known in the industry. Quarter wave coatings are known as the simplest and lowest cost option for reducing reflection since only a single layer is required. The term xe2x80x9cquarter wavexe2x80x9d applies to the optical thickness of the coating relative to the wavelength of light of interest as shown in Equation 1.
n2t=xcex/4xe2x80x83xe2x80x83Equation 1
The reflection (%) that occurs when light is incident on a quarter wave coated optical surface is given by the equation:
Reflection (%)=100(n22xe2x88x92n0n)2/(n22+n0n)2 is zero;xe2x80x83xe2x80x83Equation 2
Where n0 is the index of refraction for the ambient air; n2 is the index of refraction of the quarter wave coating material; n is the index of refraction of the optical material; t is the thickness of the coating and xcex is the wavelength of light of interest.
Optical plastics, such as polymethylmethacrylate, polycarbonate and polystyrene, typically have refractive indices of 1.451-1.62. When combined with a quarter wave antireflection coating, such as magnesium fluoride that has an index of refraction of 1.37, the reflection is 1.2%. Considering that magnesium fluoride has the lowest index of refraction of all the optical coatings typically used, this combination represents the best that can be obtained with a quarter wave coating. To obtain lower levels of reflection, more expensive multilayer antireflection coatings are required. Consequently, antireflection coatings are either not used with plastic optics and the optical performance suffers, or multilayer coatings are used and the cost of coating becomes the dominant cost of the plastic optic.
However, an examination of Equation 2 shows there exists an opportunity to obtain zero reflection exists n22=n0n. This condition is satisfied for a magnesium fluoride coated optic if the refractive index of the plastic can be increased to approximately 1.9.
Nanoparticulate fillers have been used to increase the index of refraction of plastics. By using a filler small enough that it is well below the wavelength of visible light (400-700 nm), the filler will not scatter the light and the filled plastic can retain its transparency. High refractive index powders are available in nm particle sizes that are well below the wavelength of visible light. Dispersing high refractive index nanopowders into optical plastics at specific loadings will enable the index of the plastic to be increased to 1.9 so that the magnesium fluoride coated optical article produced will have zero reflection at a given wavelength.
There have been numerous attempts in the prior art to make antireflection coatings. For instance, U.S. Pat. No. 3,706,485, titled xe2x80x9cMulti-Layer Anti-Reflection Coatings Using Intermediate Layers Having Monotonically Graded Refractive Indexxe2x80x9d by Fawcett, et al., Dec. 19, 1972, discloses a series of quarter wave coatings to produce a broadband antireflection coating. While this coating structure provides excellent antireflection properties, the need for multiple layers makes the overall cost very high for a plastic lens.
In U.S. Pat. No. 3,984,581, titled xe2x80x9cMethod For The Production Of Anti-Reflection Coatings On Optical Elements Made Of Transparent Organic Polymersxe2x80x9d by Dobler et al., Oct. 5, 1976, a method is disclosed for applying antireflection coatings to plastic optical elements without thermal treatment. While the method disclosed does improve the manufacturing process for applying antireflection coatings to plastic optics, it does not improve the performance of the antireflection coatings themselves.
In U.S. Pat. No. 4,237,183, titled xe2x80x9cProcess For The Surface Treatment Of A Synthetic Resin Lens And The Product Thereofxe2x80x9d by Fujiwara et al., Dec. 2, 1980, a process is disclosed for treating the surface of a plastic lens to improve the adhesion and durability of antireflection coatings such as magnesium fluoride. This patent does not discuss methods for improving the performance of the antireflection coating.
Further, PCT Application No. WO97/10527, titled xe2x80x9cStructured Index Optics And Ophthalmic Lenses For Vision Correctionxe2x80x9d by Toeppen, Filed Sep. 10, 1996, discloses the use of nanoparticles in ophthalmic lenses to increase the refractive index of the lens material such that the thickness of the ophthalmic lens can be reduced. While this patent does discloses the use of nanoparticles to modify the refractive index of plastics, it does not disclose the modification of refractive index of plastics to improve the performance of antireflection coatings. Likewise, the concept of increasing the refractive index of the plastic to a specific value as dictated by a quarter wave antireflective coating to deliver zero reflection at a specified wavelength is not disclosed.
In U.S. Pat. No. 5,910,522, titled xe2x80x9cComposite Adhesive For Optical And Opto-Electronic Applicationsxe2x80x9d by Schmidt et al., Jun. 8, 1999, the use of nanoparticles in an optical adhesive to improve the thermal stability of the adhesive is disclosed. The use of the nanoparticles in the lens material to effect reflection is not discussed.
Finally, in PCT Application No. WO009446A1, titled xe2x80x9cCompositions For Forming Transparent Conductive Nanoparticle Coatings And Process Of Preparation Thereforxe2x80x9d by Aikens, et al., Filed Aug. 16, 1999, the incorporation of nanoparticles into a coating to form a transparent conductive coating is disclosed. Antireflection attributes of the coating are not discussed.
Therefore, a need persists in the art for a method of making an antireflection article of manufacture having zero percent reflection enabling improved optical performances.
It is, therefore, an object of the invention to provide a method of manufacturing antireflection articles having zero percent optical reflection at a specified wavelength.
Another object of the invention is to provide a method of manufacturing optically modified materials having a nanoparticulate dispersion for improved optical performance.
It is a feature of the present invention to disperse a nanoparticulate filler into a host material and coat the resulting product with an antireflection coating which produces a surface with zero percent optical reflection.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, an antireflection article of manufacture comprises a polymeric host material, a nanoparticulate filler dispersed in said polymeric host material forming an optically modified material and a quarter wave coating layer coated on said optically modified material to form said antireflection article having a percent reflection of zero as defined by the equation
Reflection (%)=100(n22xe2x88x92n0n)2/(n22+n0n1)2=0;
Wherein n0 is the index of refraction for ambient air; n2 is the index of refraction of said quarter wave coating layer; and, n1 is the index of refraction of said optically modified material.
The present invention has numerous advantages over prior art developments, including: articles made with the optically modified materials of the invention cost considerably less to coat while having far superior optical properties; the index of refraction of the host material can be readily modified resulting in less complex optical system designs and substantial cost reductions of the overall optical system; and, high loadings of nanofillers could improve adhesion and durability of optical coatings in general.